Upvote:2
No, lines 12 and 21 do not go to Brussels-South, as you can see on the network map. This sketch lists all the stops and explains the relationship between line 12 and line 21.
Other ways to reach the airport are detailed on the airport's website. From Brussels-South, you could try to catch a train or find some other way to reach the European quarter and take the bus from there (e.g. from stop “Schuman”, which is also close to a train and a metro station by the same name). Note that some trains do call at other train stations in Brussels so getting off at Brussels-South might not always be the best choice if you are coming by train.
Upvote:2